The maximum distances optical signals can travel through optical fiber before degrading to the point of being undetectable by a receiver is limited by, among other things, power loss or attenuation caused by dispersion, scattering, absorption and bending. Optical amplifiers are employed to reduce or minimize power loss, especially in long haul systems, i.e., about 200 to 600 km, and ultra-long haul (ULH) systems, i.e., greater than about 2,000 km.
Transmission systems may include a series of optical amplifiers periodically spaced along the fiber route between the transmitter and the receiver. These amplifiers provide the necessary optical signal power.
At relatively high optical signal power, optical fiber exhibits nonlinearities such as phase shifts of the optical signal. Specifically, because modulated optical signals include different wavelengths, these different portions propagate along the transmission fiber at different velocities due to dispersion properties inherent in the fiber media. After propagation over a given distance, shorter wavelengths may overtake and become superimposed on longer wavelengths causing amplitude distortion. This is known as chromatic dispersion.
These and other factors are of particular interest in ULH telecommunication systems where, given the relatively long distances, the systems are susceptible to noise and pulse distortion. Therefore, the optical amplifiers must amplify sufficiently to raise the SNR to a level where a receiver can detect an optical signal but not be so high powered as to create intolerable nonlinearities.
Lumped rare earth doped fiber optic amplifiers such as erbium doped fiber amplifiers (EDFAs) are used in ULH optical fiber telecommunications systems. In custom systems, EDFA gains are matched to the fiber span losses to produce low noise amplified optical signals along the entire transmission path. In addition, the spans between amplifiers are preset at approximately the same lengths—between about 40 to 50 km—so that the loss per span is substantially consistent throughout the system.
Amplifying ULH terrestrial transmission systems and maintaining appropriate gain and low noise, by contrast, is somewhat more challenging. For example, ULH terrestrial systems are plagued with large span loss variations and daily and seasonal temperature fluctuations. Furthermore, unlike custom built submarine systems, terrestrial systems often have to be designed using existing fiber in the ground, unmatched and with unknown fiber characteristics. This embedded fiber base is typically non-zero dispersion shifted fiber (NZ-DSF) with a dispersion of about 2-4 ps/nm/km. Significant dispersion, therefore, may accumulate over long transmission distances.
In addition, terrestrial systems are typically designed with wide varying amplifier spacings of about 30 to 110 km. The associated span loss is very high and inconsistent. In ULH systems, the longer spans generally cause increased noise accumulation. Similarly, the nonlinearities limit the amount of power that can be launched into the next NZ-DCF span. This complicates the EDFA design and may potentially degrade performance. That is, in an attempt to minimize costs, terrestrial systems typically attempt to use a single, generic EDFA design throughout the entire system, notwithstanding the loss and nonlinearity variations from one span to the next.
Therefore, there is a need for a system and method that account for these variations in the ULH terrestrial systems and provide for optimum launch power and noise performance. There is a further need for an terrestrial system that behaves like a custom built ULH submarine system, where, for example, the input power to each EDFA is consistent throughout the system regardless of the output from the previous EDFA stage and the type and length of each span.